1. Field of the Invention
The present invention is in the field of electrical power production from steam flashed from high temperature geothermal brines which have high dissolved chloride and silica content.
2. Description of the Prior Art
Some high temperature geothermal brine resources are known which contain very large amounts of geothermal energy, but which until fairly recently had not been usable for the commercial production of electrical power because of a high dissolved silica content, and which also have a very large chloride salt content. Thus, in the Imperial Valley of California, the Salton Sea Known Geothermal Resources Area (KGRA), otherwise known as the Salton Sea Geothermal Anomaly, is estimated to have approximately 3,400 MW.sub.e of geothermal energy available for the generating of electrical power, which is believed to be self-regenerating by percolating waters. The KGRA geothermal resource is estimated to be a greater energy reserve even than the oil reserves on the North Slope of Alaska. A similar geothermal resource is the Brawley geothermal field which is also in the Imperial Valley of California. Development of these large geothermal resources was almost completely blocked until relatively recently by the high dissolved silica content, which precipitated out in vessels and piping in power production plants to the extent of up to about 42 inches per year of scaling. This problem was resolved by Magma Power Company, now of Rancho Bernardo, San Diego County, California, by flashing the geothermal brine to steam for generating electrical power in a series of first high pressure and then low pressure flash crystallizers in which the dissolved silica was precipitated out on a vast silica seed particle area rather than on surfaces of flash vessels and associated piping and valves. The use of flash crystallizers for this purpose was first taught in the Featherstone U.S. Pat. No. 4,429,535, while retrieval in reactor clarifiers of the silica seed particles useful for the purpose was taught in the VanNote U.S. Pat. Nos. 4,302,328 and 4,304,666. Applicant's U.S. Pat. No. 4,665,705 teaches an improvement over the Featherstone patent disclosure in which the flash crystallizers are made more effective by the use of external draft tubes which produce brine recirculation motive power for multifold recirculation of the geothermal brine through the flash crystallizers to provide time for silica precipitation on the silca seed particles.
Despite the foregoing solution to the silica precipitation problem, there nevertheless remained serious problems of turbine fouling and deterioration, and demister plugging and corroding in the flash crystallizers, with resulting high losses in power production revenue and high capital replacement costs in geothermal electrical power production plants located in high temperature geothermal brine regions such as the Salton Sea Geothermal Anomaly, because of entrained chlorides, silica particulates, metallic sulfide particulates and the like in the superheated steam separated from the hot geothermal brine in the flash crystallizers. Applicant has determined that steam which is separated in flash crystallizers from geothermal brine in the Salton Sea Geothermal Anomaly region contains on the order of about 500-1,000 parts per million of chlorides, together with a substantial amount of solid particulates including silca and metallic sulfide particles. These entrained materials carried with the separated steam are in the form of chloride-laden brine droplets or foam flecks which carry the solid particulate materials with them. In addition to these undesirable substances carried with the separated steam, the high velocity upward flow of separated steam in the flash crystallizers tends to cause some liquid brine to swell and crawl up the walls of the flash crystallizers from the bodies of liquid brine in the lower portions of the flash crystallizers.
Components of the dissolved solids content of the hot geothermal brine include the three primary chloride salts, sodium chloride, calcium chloride, and potassium chloride, a large dissolved silca content, and metallic sulfides. A large amount of the silica content precipitates out on the silica seed particles provided from the reactor clarifier or clarifiers as heat is extracted from the brine by the flashing of steam in the flash crystallizers, and some of the chlorides and metallic sulfides precipitate with evaporation of superheated steam from the brine in the flash crystallizers. Prior to the present invention, some of these materials have inevitably become entrained in the upwardly flowing flashed steam in the flash crystallizers and partially separated out from the steam by demisters located in the upper portions of the flash crystallizers, but also partially delivered to the steam-driven turbine or turbines. This has resulted in severe problems with both the demisters and turbines.
A geothermal power plant at the Salton Sea geothermal field having a net electrical power output on the order of from about 30 to about 50 megawatts will normally have two parallel hot geothermal brine flow systems, each of which includes a tandem sequence of a high pressure flash crystallizer and a low pressure flash crystallizer for separating steam from the geothermal brine flow streams for delivery to respective high and low pressure steam turbine stages. Each of these four flash crystallizers is normally provided in its upper portion with a demister device for the purpose of removing entrained brine droplets and particulates contained therein from the separated steam before the steam passes to the turbine, in order to protect the turbine or turbines. Prior to the present invention, such demisters have proven to be incapable of removing a sufficient amount of the brine and contained particulates entrained in the flowing steam to adequately protect the turbine or turbines, yet the quantity of entrained brine and particulates has been so large as to rapidly foul and scale the demisters, back-pressuring the flash crystallizers and seriously reducing the power generating capacity of the plant.
Prior to the present invention, such fouling of the flash crystallizer demisters has been so severe as to require that the demisters be replaced approximately every two to three weeks, with substantial plant down-time which was usually several days. In addition to the capital cost of replacing the demisters, such down-time resulted in a large revenue loss, as for example on the order of about $75,000 per day for a net 40,000 megawatt plant.
Even with the demisters, prior to the present invention a sufficient amount of brine droplets and particulates flowed with the separated steam to the turbine or turbines to cause damage to the turbines, particularly to high pressure turbine stages. Such damage included deterioration of the turbine blades, and also some shroud cracking and breaking on the turbine rotor assemblies. This, of course, involved substantial additional capital replacement costs and revenue loss from down-time.